The present invention relates generally to varying impedances to improve the isolation between transmit and receive circuits in wireless transceivers, and more particularly to controlling the impedances using shunt switches.
Wireless circuits are rapidly becoming ubiquitous, appearing in a number of electronic applications such as desktop and laptop computers, handheld devices, camcorders, inventory control devices, and myriad other devices. At the same time, pricing pressures have increased as these circuits are seen less as a feature and more as a requirement. Accordingly, it is desirable to reduce costs, for example by reducing the number of components necessary to implement a wireless solution. One such component that may be removed is a transmit and receive switch.
Wireless circuits send data using a transmitter and receive data using a receiver. When a wireless circuit is transmitting, the receiver is still present. Unfortunately, any loading the receiver places on the transmitter reduces the transmitted power, thereby reducing transmitter range. Similarly, when the wireless circuit is receiving, the transmitter remains in place and any received power absorbed by the transmitter reduces the effective input received signal strength.
To provide the necessary isolation between receiver and transmitter, conventional wireless circuits make use of a transmit and receive switch to disconnect the transmitter when data is received and the receiver when data is transmitted.
Accordingly, conventional circuits insert a switch in series into either or both the receive and transmit paths. These switches isolate the receiver when transmitting and isolate the transmitter when receiving. Unfortunately, these switches are not ideal; that is, they include series resistances, capacitances, and inductances, and other parasitic components. Because of this, the switches reduce system performance. For example, some of the transmitted power is absorbed by the switch, reducing the power transmitted over the antenna. Similarly, when a signal is received, some of the received signal strength is lost in the switch.
To reduce these losses, the switches are typically off-chip devices designed for this purpose, which adds to the cost of the circuit.